1. Field of the Invention
This invention relates to the field of transfer cases for motor vehicles; more particularly it pertains to control strategies and systems for such transfer cases.
2. Description of the Prior Art
A transfer case is a device located in a motor vehicle drive line between the output of a geared power transmission and front and rear driveshafts for transmitting power to the wheels. A transfer case may include a planetary gear set that produces both a high range, in which the transfer case output is driven at the same speed as the input, and a low range, in which the output is driven slower than its input speed. The 4×2 and 4×4 states of the transfer case are usually selected manually by the vehicle operator by operating a lever or switch. A first position of the lever causes a range selection device in the transfer case to direct power from the transmission output to a rear drive axle, the 4×2-drive mode. A second position of the lever causes the transfer case to direct power to both a front drive axle and a rear drive axle, the 4×4-drive mode.
In many commercially available drive systems, the front and rear wheels share power output by the transfer case equally. Several electronically controlled, on-demand control systems are known, but they have comparatively unsophisticated mechanical systems and controls. Generally such systems minimize slip by simply locking the system instead of continually controlling the division or split of input torque that is transmitted to the front and rear wheels.
An alternative to the speed equalization between front and rear wheels is provided by a transfer case having a center differential mechanism. Such a differential includes a planetary gearset having an input, such as a sun gear driven by the transmission output; a first output, a carrier connected to the rear driveshaft; and a second output, a ring gear connected to the front driveshaft. The differential mechanism continually divides torque at a fixed ratio between the front and rear wheels, perhaps 35% of torque to the front wheels and 65% to the rear wheels. But the differential mechanism provides no variation of the torque division to improve vehicle handling under certain drive conditions. Furthermore, it is a well known limitation of an open differential that little drive torque can be sent to an axle with traction when the other axle loses grip.
To overcome this deficiency in adapting performance to drive conditions, a device such as a viscous coupling is sometimes provided in a transfer case. A viscous coupling, located in parallel with the front and rear driveshafts, or the first and second outputs of a center differential, operates to mutually connect or couple the driveshafts in proportion to the speed difference between them. It produces this effect by shearing a viscous fluid located between closely spaced sets of plates, one set of plates driven by the front driveshaft and a second set of plates driven by the rear driveshaft. Variations in the speed difference of the sets of plates increase the magnitude of the forces tending to maintain the plates at the same speed. The coupling dissipates a portion of the output power in the process of synchronizing the speeds.